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Doping Golden Buckyballs Cu@Au16 and Cu@Au17 Cluster Anions.

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Angewandte
Chemie
DOI: 10.1002/ange.200700060
Gold Cages
Doping Golden Buckyballs: Cu@Au16 and Cu@Au17 Cluster
Anions**
Lei-Ming Wang, Satya Bulusu, Hua-Jin Zhai, Xiao-Cheng Zeng,* and Lai-Sheng Wang*
The discovery of the unique catalytic effects of gold nanoparticles on oxide substrates[1] has stimulated a flurry of
research into the structures and properties of free gold
nanoclusters, which may hold the key to elucidating the
catalytic mechanisms of supported gold clusters. One of the
most remarkable results has been the discovery of planar gold
cluster anions (Aun ) of up to twelve gold atoms and the 2D to
3D transition for clusters with n larger than 12.[2–4] Among
larger gold clusters, Au20 has been found to be a perfect
tetrahedron.[5] A more recent study of the structures of Aun
cluster anions in the medium size range (n = 15–19)[6] has
shown that clusters with n = 16–18 possess unprecedented
empty cage structures. In particular, the Au16 cluster anion
has an interesting tetrahedral structure with an inner diameter of about 5.5 0 and can be compared to the fullerenes
(buckyballs). Although Au32 was first suggested to be a “24carat golden fullerene”,[7, 8] subsequent studies showed that
the Au32 ion is in fact a low-symmetry compact 3D
structure.[9] Other larger gold cage clusters have also been
proposed computationally,[10, 11] but none has been observed
or is expected to be the global minimum. The cage structures
of the cluster anions Au16 and Au17 have recently been
confirmed by electron diffraction[12] and thus they are the first
experimentally confirmed and the smallest possible gold
cages. The large empty space inside these cage clusters
immediately suggested that they can be doped with a foreign
atom to produce a new class of endohedral gold cages[6]
analogous to endohedral fullerenes.[13, 14]
A gold cage containing a central atom was first predicted
for a series of icosahedral clusters M@Au12 (M = W, Ta , Re+)
based on the 18-electron rule[15–17] and was subsequently
confirmed experimentally.[18, 19] However, since Au12 itself
does not possess a cage structure, the dopant atom with the
appropriate electron count must play an essential role in
holding the cage together. Bimetallic gold clusters have been
studied experimentally[20–24] as they offer new opportunities to
fine-tune the electronic and structural properties of gold
nanoclusters. Following the discovery of the hollow gold
cages,[6] two recent theoretical studies have appeared concerning doping them with a foreign atom.[25, 26] Since the
parent Au16 and Au17 cluster anions are empty cages, many
different types of atoms could be used as dopants to form new
endohedral gold clusters.[6] Herein we report the first
observation and characterization of Au16 and Au17 doped
with a Cu atom (Cu@Au16 and Cu@Au17 ) by both photoelectron spectroscopy (PES) and density functional theory
(DFT) calculations.
The experiment was performed in a magnetic-bottle PES
apparatus equipped with a laser vaporization supersonic
cluster source.[27] Figure 1 shows the spectra of the ions
[*] L.-M. Wang, Dr. H.-J. Zhai, Prof. Dr. L.-S. Wang
Department of Physics, Washington State University
2710 University Drive, Richland, WA 99354 (USA)
and
Chemical & Materials Sciences Division
Pacific Northwest National Laboratory
MS K8-88, P.O. Box 999, Richland, WA 99352 (USA)
Fax: (+ 1) 509-376-6066
E-mail: ls.wang@pnl.gov
S. Bulusu, Prof. Dr. X.-C. Zeng
Department of Chemistry and Center for Materials and Nanoscience
University of Nebraska, Lincoln, NE 68588 (USA)
Fax: (+ 1) 402-472-9402
E-mail: xczeng@phase2.unl.edu
[**] The experimental work done at Washington was supported by the
U.S. NSF (CHE-0349426) and the John Simon Guggenheim
Foundation and performed at the EMSL, a national scientific user
facility sponsored by the U.S. DOE’s Office of Biological and
Environmental Research and located at PNNL, operated for DOE by
Battelle. The theoretical work done at Nebraska was supported by
the DOE Office of Basic Energy Sciences (DE-FG02-04ER46164), the
NSF (CHE-0427746, CHE-0314577, DMI-0210850), the John Simon
Guggenheim Foundation, and the University of Nebraska-Lincoln
Research Computing Facility.
Angew. Chem. 2007, 119, 2973 –2976
Figure 1. Photoelectron spectra of the cluster anions CuAu16 and
CuAu17 , compared to Au16 and Au17 ; see text for details.
CuAu16 and CuAu17 along with those of the parent gold
clusters.[6] Let us first focus on the CuAu16 ion (Figure 1 b),
whose PE spectrum is remarkably similar to that of its parent
gold cluster Au16 (Figure 1 a). The first three features (X, A,
and B) and the gap between B and C (Figure 1) are all very
similar in the PE spectra of both systems, except that the
intensity of the ground state band (X) is greater for the doped
cluster and its electron-binding energies are slightly higher
(Table 1). The similarity between the spectra of these two
2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2973
Zuschriften
Table 1: Experimental adiabatic (ADE) and vertical (VDE) detachment
energies for the doped cluster anions Cu@Au16 and Cu@Au17 along
with those for the Au16 and Au17 anions and the calculated VDE values.
All energies are given in electron volts.
Anion
ADE
Au16 [a]
Cu@Au16 (Cs)
Au17 [a]
Cu@Au17 (C2v)
Experimental
VDE
3.99 0.03
4.12 0.05
4.03 0.03
3.16 0.06
4.03 0.03
4.16 0.03
4.08 0.03
3.23 0.03
Theoretical
VDE
4.179
4.092
4.053
3.155
[a] Taken from ref. [6].
species suggests that the Cu doping does not alter the
geometric and electronic structures of the Au16 cluster
anion significantly, which is only possible if Cu is trapped
inside the Au16 cage. The Au16 cluster anion itself is unique
and its PE spectrum does not exhibit an energy gap similar to
that for other even-sized gold clusters in this size range.[4, 6, 28]
The high electron-binding energies and the lack of an energy
gap suggest that the neutral Au16 cluster is open-shell and
probably has two unpaired electrons (a triplet state).[6] This
means that two extra electrons would be required to reach a
closed-shell 18-electron Au162 ion, which is also borne out by
a recent theoretical study.[25] Because of the high electron
affinity of Au, the Cu atom can be viewed as donating an
electron to the gold cage in CuAu16 , which gives rise to a
closed-shell and stable Au162 dianion. Thus, the CuAu16
cluster anion can best be viewed as Cu+@Au162 .
The spectrum of the doped cluster anion CuAu17 is also
very similar to that of the parent gold cluster Au17 ; except
that there is one low-binding-energy peak followed by a large
energy gap in the spectrum of the Cu-doped cluster (Figure 1 c,d). The five peaks between 4 and 5 eV in the spectrum
of the CuAu17 cluster anion (labeled A–E in Figure 1 d) are
remarkably similar to the five characteristic low-bindingenergy features in the spectrum of the parent gold cluster
(Au17 ; Figure 1 c). This spectral similarity again suggests that
the Cu dopant induces very little structural change in the
Au17 cage except that it donates one electron. Au17 is a
closed-shell species with 18 valence electrons,[6] therefore the
extra electron is expected to enter its LUMO and give rise to
the low-binding-energy peak (X) in the PE spectrum of the
CuAu17 cluster anion (Figure 1 d). All these observations
again imply that Cu stays in the center of the Au17 ion cage
(Cu+@Au172 ) and does not perturb the electronic and
geometric structures of the cage significantly.
We carried out theoretical studies to confirm these
observations (see Experimental Section). The results revealed
that the endohedral Cu@Au16 and Cu@Au17 cluster anions
are overwhelmingly favored over any other structure with the
Cu atom on the outside of the cage. Figure 2 shows the
simulated PE spectra for two endohedral structures each for
the Cu@Au16 and Cu@Au17 cluster anions along with those
of the parent clusters.[6] In one structure, the Cu atom is
located in the center of the cages and in the other it is
displaced slightly from the center. The energy differences
between the two isomers are very small and their simulated
PE spectra are also very similar to each other. The endohedral
2974
www.angewandte.de
Figure 2. Simulated photoelectron spectra for two endohedral structures each for Cu@Au16 and Cu@Au17 along with those for Au16
and Au17 .
Cu@Au16 cluster anion with Cu in the center has Td
symmetry with a triply degenerate HOMO, which gives rise
to the first band in the simulated PE spectrum (Figure 2 b). In
the structure in which the Cu atom is displaced from the
center, the Cu@Au16 cluster anion has Cs symmetry and the
triply degenerate HOMO is split, which gives rise to the
doublet peaks (X and A) in the simulated PE spectrum
(Figure 2 c) and is in perfect agreement with the experimental
spectrum (Figure 1 b). The simulated PE spectra of both the
C2v and Cs structures of the Cu@Au17 cluster anion are
similar to each other and are both in good agreement with the
experimental spectra, thereby suggesting that the Cu atom in
the center of the Au17 anion cage might be somewhat
fluxional.[29]
The calculated vertical detachment energies (VDEs) for
the Cu@Au16 and Cu@Au17 cluster anions are also in good
agreement with the experimental values (Table 1). Overall,
the excellent agreement between theory and experiment
confirms the endohedral structures of these Cu-doped gold
cages unequivocally. It is important to note, however, that the
Au16 and Au17 cages are not distorted significantly from
those of the parent clusters even in the low-symmetry
structures.
Figure 3 shows the frontier orbitals of the two endohedral
clusters. The electron densities are clearly dominated by the
cages, with little contribution from the central Cu atom; this is
consistent with the description of Cu@Au16 and Cu@Au17 as
Cu+@Au162 and Cu+@Au172 , respectively.[30] The chargetransfer interactions between the cage and its dopant are also
reminiscent of endohedral fullerenes[13, 14] and are consistent
with the strongly ionic character of the diatomic molecule
CuAu.[31]
Doping gold clusters could be a powerful way to tune their
chemical and physical properties,[24, 32, 33] and the results
reported herein suggest that a new class of endohedral gold
cages is indeed viable. In these examples the cage structures
of Au16 and Au17 cluster anions are maintained simply by
changing the dopants, which is reminiscent of the behavior of
2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2007, 119, 2973 –2976
Angewandte
Chemie
PBEPBE functional and the LANL2DZ basis set. The first VDE was
calculated by the energy difference between the anion and neutral
clusters at the anion geometry. VDEs to higher detachment channels
were computed by adding the occupied orbital energies relative to the
HOMO to the first VDE. The simulated PE spectra were obtained by
fitting the computed VDEs with Gaussian functions with a width of
0.04 eV. The isomer structures that gave the best match between
simulated and measured spectra were identified and are shown in
Figure 2. Note that the Td isomer of the Cu@Au16 ion shown in
Figure 2 b has four imaginary frequencies (owing to Jahn–Teller
distortion), whereas the Cs isomer of the Cu@Au16 ion shown in
Figure 2 c has no imaginary frequencies and represents a global
minimum.
Received: January 5, 2007
Published online: March 12, 2007
.
Keywords: cluster · copper · electronic structures · gold ·
photoelectron spectroscopy
Figure 3. The HOMO and LUMO of cluster anions Cu@Au16 (Cs) and
Cu@Au17 (C2v).
endohedral fullerenes.[13, 14] It would be particularly interesting to dope transition-metal atoms inside these gold cages to
create magnetic gold clusters as these may exhibit new,
physical, chemical, and catalytic properties that are distinct
from the pure gold clusters.
Experimental Section
Photoelectron spectroscopy: The CuAu16 and CuAu17 cluster anions
were produced by laser vaporization of an Au/Cu composite disk
target containing about 7 % Cu. Negatively charged clusters were
extracted from the cluster beam and analyzed with a time-of-flight
mass spectrometer.[27] The clusters of interest were mass-selected and
decelerated before being intercepted by a 193-nm laser beam from an
ArF excimer laser for photodetachment. The Cu content in the Au/Cu
target was carefully adjusted to minimize multiple Cu doping in the
AuxCuy clusters and provide the AuxCu series as the dominant
doped species in order to achieve clean mass-selection for the
CuAu16 and CuAu17 anions. Photoelectron time-of-flight spectra
were calibrated against the known spectra of Au and converted into
binding energy spectra by subtracting the kinetic energy spectra from
the photon energy. The resolution of the magnetic-bottle PE
spectrometer was DE/E 2.5 % (i.e., about 25 meV for 1-eV
electrons).
Calculations: We performed global-minimum searches using the
basin-hopping method[34] for the anionic gold clusters Aun (n = 16,
17) doped with a Cu atom. We combined the global search method
directly with DFT calculations.[6, 25, 35] After each accepted MonteCarlo move a geometry minimization was carried out using a DFT
method with a gradient-corrected functional, namely the Perdew–
Burke–Enzerhof (PBE) exchange-correlation functional,[36] implemented in the DMol3 code.[37] Top low-lying isomers were collected
and reoptimized using the PBEPBE functional and LANL2DZ basis
set, as implemented in the Gaussian 03 package.[38] Frequency
calculations were also carried out to assure the optimized structures
were local minima. Finally, PE spectra were calculated using the
Angew. Chem. 2007, 119, 2973 –2976
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